1. Field of the Invention
The present invention relates to catalyst compositions comprising oxides of rare earth metals and palladium which are suitable for the combustion of gaseous carbonaceous fuels such as natural gas or methane, and to a process for catalytic combustion of such fuels using the catalyst compositions.
2. Related Art
Complexes of palladium oxide and rare earth metal sesquioxides are known, as shown by an article by C. L. McDaniel et al, "Phase Relations Between Palladium Oxide and the Rare Earth Sesquioxides in Air", Journal of Research of the Natural Bureau of Standards - A. Physics and Chemistry, Vol. 72A, No. 1, January-February, 1968. Pages 27-37 describe complexes of PdO and rare earth metal oxides. Specifically, the paper describes the study of equilibrium phase relations in an air environment between PdO and each of the following sesquloxides: Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3, La.sub.2 O.sub.3, Eu.sub.2 O.sub.3, Gd.sub.2 O.sub.3, DY.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Y.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3 and Lu.sub.2 O.sub.3. The experimental procedure employed is described In section 3 at page 28 and describes mixing various combinations of PdO and the rare earth metal oxides and subjecting the mixture to preliminary heat treatments for a minimum of 18 hours at 770.degree. C. and then at 780.degree. C. After each heat treatment the materials were examined by x-ray diffraction techniques and following the preliminary heat treatments portions of each batch were fired at various temperatures, typically lying between 1000.degree. C. and 3000.degree. C. as set forth in TABLE I, pages 29-33. Among other findings, the paper notes (in the summary of section 4.3 at page 34) the dissociation temperature of PdO in air at atmospheric pressure to be 800.degree. C..+-.5.degree. C., and that palladium oxide reacts with a number of the rare earth metal oxides to form binary compounds. What is described as the pseudobinary system Nd.sub.2 O.sub.3.sup.. PdO is said to exemplify the typical type of reaction and three binary oxide compounds of, respectively, 2:1, 1:1 and 1:2 ratios of Nd.sub.2 O.sub.3 :PdO are disclosed, viz, 2Nd.sub.2 O.sub.3.sup.. PdO; Nd.sub.2 O.sub.3.sup.. PdO and Nd.sub.2 O.sub.3.sup.. PdO. (The compound 2Nd.sub.2 O.sub.3.sup.. PdO may of course be written as Nd.sub.4 PdO.sub.7.) Analog compounds are noted for the Sm.sub.2 O.sub.3.sup.. PdO, Eu.sub.2 O.sub.3.sup.. PdO and La.sub.2 O.sub.3.sup.. PdO systems, with only the 2:1 and 1:2 compounds occurring in the latter system. However, it was noted that other rare earth oxide-palladium oxide combinations did not react in the solid state. There were combinations of PdO with, respectively, Ho.sub.2 O.sub.3, Y.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3 and Lu.sub.2 O.sub.3. (See the Abstract at page 27, and the last sentence on page 35.)
Another article, by A. Karo et al, "Lanthanide B-Alumina Supports For Catalytic Combustion Above 1000.degree. C.", Successful Design of Catalysts, 1988 Elsevier Science Publishers, pages 27-32, describes the preparation of support materials consisting of lanthanide oxides and alumina for use as combustion catalysts. The article states that endurance tests on methane combustion performed at 1200.degree. C. proved that a Pd catalyst supported on lanthanum beta-alumina has good resistance to thermal sintering (page 32).
An English language abstract of Japanese Patent Publication J 63088041 (1988) TsuJ et al, Tanaka Kikinzoku Kogyo discloses a catalyst consisting of a ceramic support composed of alumina on whose surface Pd and/or PdO is supported on a layer of intermetallic PdM or PdMO wherein M is a rare earth metal. The disclosed use is for catalyzing combustion of fuel without generation of nitrogen oxides. Chemical Abstracts (CA 109 11 32 93k) identifies the patentee as Tanaka Noble Metal-Industrial Co. Ltd. and describes the methane combustion catalyst as Pd-Pd/LaO.sub.2 on gamma-alumina and reports that this catalyst cornbusted natural gas at an inlet temperature of 355.degree. C. with 100% efficiency after 1 hour and with 99.98% efficiency after 1,000 hours.
U.S. Pat. No. 4,893,465 issued to Robert J. Farrauto et al describes a process for the catalytic combustion of carbonaceous materials, such as natural gas or methane, using a palladium oxide containing catalyst. In the process, the palladium oxide catalyst for the catalytic combustion Is subjected to temperatures in excess of the decomposition temperature of palladium oxide to metallic palliadium, the latter being inactive for catalysis of the combustion reaction. At atmospheric pressure the decomposition temperature of PdO is at least about 800.degree. C. The stated improvement in the process of the Patent comprises restoring catalytic activity by lowering the temperature of the catalyst to a regenerating temperature, i.e., a temperature at which Pd is oxidized to PdO, which in air at atmospheric pressure is from about 530.degree. C. to about 650.degree. C., and maintaining the temperature within that range until desired catalytic activity is achieved by re-oxidation of catalytically inactive Pd to PdO. The examples of the Patent utilize PdO/Al.sub.2 O.sub.3 as the catalyst.